The index of leading economic indicators dropped 0.1 percent in June while revisions of the May index wiped out a previously reported decline, the Commerce Department said yesterday.

Separately, the Agriculture Department also reported that prices paid to farmers for their crops rose 0.8 percent in July, raising the prospect of higher food prices, particularly late this year and in 1980.

The irregular path of the leading indicators, movements of which generally foreshadow changes in ecomonic activity, suggests that the recession most economists say has begun may be a mild one, analysts said.

Robert Gough of Data Resources Inc., an economic consulting firm, said the index "is giving mixed signals . . . but the details show that the business community is now taking seriously the earlier signs that the consumer is staying hoem and paying off his debt."

The Commerce Department revisions now show that the index has declined in five of the past 12 months, but never for two months in a row. The preliminary numbers for June place the index below the level first reached in April 1978.

On a quarterly basis, the average for the first three months of this year dropped 0.2 percent from the fourth-quarter level, and the second quarter fell another 1.7 percent.

Contributing most to the June drop of 0.1 percent was an increase in lay-offs in manufacturing industries. A drop in new orders for consumer goods and materials, adjusted for inflation, was close behind in pushing the index down.

Five of the 10 indicators now available rose, however, with an increase in new orders for business plants and equipment, also adjusted for inflation, the most significant.

The surge in farm prices began early in June after repeated reports that this year's Russian grain crop was in trouble. The Soviets already have bought 14.8 million metric tons of grain for delivery by October, and they are expected to make large purchases of wheat, corn and other grain next year.

Wheat prices rose 23 cents a bushel in July to $3.95, $1.14 higher than a year ago. Corn jumped 24 cents to $2.73 a bushel.

Higher grain prices usually affect retail food prices with a significant lag. However, consumers are already beginning to feel a big increase in some other farm prices.

The price of new-crop potatoes nearly quadrupled, from $1.22 to $4.57 per hundred pounds. Grapefruit, apple and strawberry prices also rose.

Live cattle prices declined, however, for the third month in a row, indicating that beef prices in supermarkets could decrease.

Altogether, July farm prices were 14 percent higher than in July 1978, and the Agriculture Department keeps raising its estimate for the total increase in retail food prices this year.

Last week the department's chief economist, Howard Hjort, said he expects retail food prices for all of 1979 to average "close to 11 percent" higher than last year. CAPTION: Graph, Leading Economic Indicators, The Washington Post